A breathing assistance device typically delivers pressurized breathable gas to a patient via tubing called a “patient interface” or “breathing circuit.” The breathable gas typically includes air and/or one or more additional or supplemental gases (e.g., supplemental oxygen mixed with air). The breathing assistance device typically increases the pressure in the breathing circuit to push the breathable gas into the lungs for inspiration, and reduces the pressure in the breathing circuit to allow gases in the lungs to be expired and vented to the atmosphere. Typically, one or more breathing assistance device parameters may be determined and/or adjusted prior to and/or during operation. Such parameters may include, for example, the mode of ventilation (e.g., CMV (controlled mandatory ventilation), SIMV (synchronized intermittent mandatory ventilation), CPAP (constant positive airway pressure), or bi-level CPAP); the patient's tidal volume (the volume of gas inspired with each breath); the respiratory rate (the number of breaths per minute (BPM)); and/or the O2 concentration, flow rate, airway pressure, and/or minute volume (the volume inspired and expired in one minute) of breathable gas delivered to the patient.
During inhalation, gas inhaled by a patient typically passes through the patient's airway and enters the alveoli of the patient's lungs, where pulmonary gas exchange may take place. Pulmonary gas exchange is driven by passive diffusion, whereby highly-concentrated oxygen moves from the alveoli to the patient's blood stream, which typically has a lower oxygen concentration due to the continuous consumption of oxygen in the body. Conversely, the patient's metabolism may produce a higher concentration of carbon dioxide than that of the alveoli, causing diffusion of carbon dioxide from the blood stream to the alveoli, which may then exhaled by the patient.
As mentioned above, the breathable gas delivered by a breathing assistance device typically includes air and/or one or more additional or supplemental gases. In many applications, supplemental oxygen may be mixed with pressurized air to deliver a desired concentration of oxygen to a patient. Alternatively, pure oxygen may delivered to a patient.
Because gas inhaled during the beginning of an inhalation phase is more likely to reach the alveoli (as compared with gas inhaled later during the inhalation phase), oxygen present in such gas is more likely to diffuse into the bloodstream. Gas inhaled during the later portions of the inhalation phase may not diffuse as effectively into the alveoli, and thus oxygen present in such gas may not enter the bloodstream, and may instead be expired by the patient. Accordingly, in applications in which supplemental oxygen is used, the supplemental oxygen delivered during latter portions of an inhalation phase may be, in effect, wasted.